"In wilderness I sense the miracle of life, and behind it our
scientific accomplishments fade to trivia." Charles A. Lindbergh.

Pacemakers have evolved over a period of time trying to mimic the
normal response rates, conduction and activation characteristics,
though are still far from what nature has bestowed upon us. Better
understanding of cardiac physiology and hemodynamics has led to current
available pacing technology and we do recognize now that to achieve
physiological pacing we should have an appropriate heart rate response,
ventriculo-ventricular (VV) synchronization and atrio-ventricular (AV)
synchronization.

Patients receiving rate responsive pacemakers for sinus node
dysfunction, in spite of using various sensors and rate response
algorithms, [1-5] still do not truly have an appropriate heart rate
response, especially in absence of physical stress. There is a need to
develop sensors, based on which an algorithm can be developed to
achieve a heart rate response, which truly mimics to what a normal
sinus node would behave in response to both physical and mental stress.
In patients with heart block who have atrial sensing based ventricular
pacing, the heart rate response remains appropriate if the sinus node
is normal.

Right ventricular (RV) pacing represents a non-physiological activation
of the heart causing wide QRS (left bundle branch block) with
electrical and mechanical VV dyssynchrony.[5] Higher percentage of
ventricular pacing in patients with intact AV node has been found to be
associated with increased incidence of atrial fibrillation and heart
failure on follow up. [6-10] Algorithms to prevent ventricular pacing
are effective in reducing unnecessary ventricular pacing in patients
with normal AV conduction and sick sinus syndrome. However these
algorithms cannot be applied to patients with advanced heart block in
which there is need for mandatory ventricular pacing. To avoid
detrimental effects of VV synchrony alternate site RV pacing [11-15]
and biventricular pacing have been described. [16,17] Alternate site
pacing studies have shown mixed results. [11-15] Left sided lead
placement, non-physiological epicardial pacing and procedure and pacing
related complications with the higher overall cost involved in doing
biventricular pacing procedure represents a significant limitation for
advising it as a routine. VV dyssynchrony possibly would remain a
limitation in achieving total physiological pacing till further
conclusive evidence of newer pacing methods is demonstrated.

Optimal AV interval at rest ranges from 100 to 150 milliseconds. In
normal individuals the AV interval shortens with increased heart rate
during exercise in a predictable and linear fashion. Most pacemakers
have a programmable shortening of AV delay at higher rates, the
hemodynamic benefits of which have not yet been shown. [1] The aim of
optimizing AV delay in patients with heart failure is to increase
diastolic filling and at the same time maintain biventricular pacing so
as to maximize cardiac output. In patients with heart failure and LV
dysfunction even a small improvement in cardiac output, as obtained by
optimizing AV delay, may result in significant clinical improvement. AV
optimization is routinely done using echocardiographic techniques of
which Ritter's method is the most commonly used. [18] Device based
algorithms like QuickOpt is also available and is currently being
evaluated for its effectiveness in comparison to echocardiographic
methods. [19] Optimizing AV synchrony and hence AV delay is routinely
not advised in patients receiving pacemakers without heart failure.

An electrocardiogram based method to determine optimal AV interval is
described by Sorajja et al [20] in this issue of the journal, in which
P wave duration correlates with a correction factor of 1.26 with an
optimal AV interval, as determined by Ritter's method of AV
optimization on echocardiography. Such simple technique can be used for
effectively programming optimal AV delay routinely once validation by
large trials occur, so as to achieve better hemodynamics without the
need for time consuming echocardiographic techniques or till the time
echocardiographic optimization is routinely planned. This study, though
with its limitations of having a small cohort of elderly patients and
optimization evaluated only at rest, presents an attractive alternative
to echocardiography based techniques to calculate and program optimal
AV delay.

Based on echocardiographic parameters and natriuretic peptide levels,
AV delay optimization is found to be beneficial in patients with normal
LV function in short term small studies. [21-25] There exists hardly
any long term study to demonstrate benefits of routine optimization of
AV delay in patients having normal LV function and receiving pacemakers
for heart block. Hence it would be difficult to justify
echocardiography based AV optimization in all such patients. However it
seems appropriate to aim to program an optimal AV delay in all patients
receiving pacemakers, based on data from heart failure patients and
short term studies. Can the findings of this study be extrapolated for
use in AV optimization in patients treated with devices for heart
failure? Larger studies in patients with and without LV dysfunction and
heart failure would be required to validate the results of this pilot
study for incorporating it in clinical practice to achieve better long
term outcomes. We still have a long way to go before we can mimic with
pacemakers the normal electrical activity of the heart.

Adopt the pace of nature: her secret is patience - Ralph
Waldo Emerson
Look deep into nature, and then you will understand everything
better - Albert Einstein